Change isc_entropy_usebestsource() to have saner semantics:
 - If an invalid file is specified, an error will be returned instead of the
   keyboard being used.
 - If no file is specified but a random device is present, the keyboard will
   be used if there is an error opening the random device.
 - ISC_ENTROPY_KEYBOARDYES indicates that the keyboard should be the
   only device used.  Otherwise, passing '-r keyboard' is meaningless
   on a machine with a random device, since the keyboard will not be used.

Change the callers in the dnssec tools and rndc-confgen to check for the
special file "keyboard" and call isc_entropy_usebestsource() with the right set
of parameters.